The GOF is simply a numerical assessment of the quality of the match of your instrument and crystal structure model to an experimental data set. A value of 4 might be perfectly adequate or unacceptably bad - it depends on what you are trying to learn from your data. Most likely a value of 4 is indicating that there are model parameters that need to be modified to get a better fit - these might include using a more suitable structure model, a more suitable profile function or background function, thermal parameters, 2theta zero, etc. A visual inspection of the difference plot can often suggest what to try next.

Dear Bibi Safia.

A good overview of this topic can be found in this publication: https://www.researchgate.net/publication/245022853_R_Factors_in_Rietveld_Analysis_How_Good_is_Good_Enough.

In my opinion, the most important criteria are

1. Physical sense of structures and models used during the refinament.

2. Visual inspection of Rietveld Refinement.

3. Achieve the lowest possible discrepancy factors.

In principle, a good hint is to try to get Rwp and Rp as close as possible to the Re value.

Another more appropriate way would be to perform a Le Bail or Pawley refinement, and these discrepancy factors obtained are the best possible values for an ideal Rietveld refinement. These values would serve as a reference for the final Rietveld refinement to be performed.

Finally, one way of knowing that you are reaching the limit of refinement is to look at the difference between Rwp and Rp, the idea is to try to reduce it as much as possible.

Don't forget the physical sense of parameters and structure, and the problems associated with using the wrong structural model.

For the particular case of the Goof, it should ideally be close to 1. You should avoid values below 1.

Best regards

Analio Dugarte